This invention concerns archery bows and more particularly archery bows of the type having means for reducing the force required to maintain the bowstring in its full draw position, commonly referred to as a "compound bow".
An archery bow of the compound bow type is described in U.S. Pat. No. 3,486,495 to H. W. Allen. In this construction, a pair of eccentric rotary elements are mounted on either tip of the limbs of the bow member and the bowstring is wrapped around the exterior of each rotary element and connected thereto such that drawing of the bowstring rotates each rotary element about its axis of rotation. The rotary elements are also each connected to a load cable which extends from the connected rotary element alongside the bowstring to the opposite limb tip where it is anchored. The configuration of the rotary elements and the points of connection of the load cables and bowstring is such that as the elements rotate, a variable leverage is exerted by the load cables tending to bend the bow member limbs. This occurs in such a manner that the force required to maintain the bowstring in its fully drawn position is less than the maximum force required at some point during the drawing of the bowstring.
This has the distinct advantage of not requiring maximum force to be exerted by the archer to improve aim and release, and also improves the manner of launching of the arrow, since the forces applied to the arrow increase progressively with recovery motion of the bowstring.
These advantages have led to very widespread use of the principle of the Allen patent which has had a revolutionary impact on the art.
The presence of the load cables extending between the tips of the bow member, however, is a disadvantage in that it tends to interfere with the shooting of the bow since the load cables occupy the central region of the bowstring where the arrow is nocked. The presence of the load cables is also aesthetically unpleasing and particularly so when various fixtures are employed to hold the load cables out of the way as they pass through the space adjacent the arrow nocking point on the bowstring.
In U.S. Pat. No. 4,261,320 to Barna, there is disclosed a rerouting of the load cables through the handle section of the bow member by means of guide pulleys. The force exerted by the load cables is exerted in a direction generally aligned with the bow member limbs producing "longitudinal" flexing of the bow limbs, in addition to conventional transverse flexing of the bow limbs produced by the bowstring directly. Such longitudinal flexing is advantageous in that this mode of loading of the limbs stores more energy for a given degree of bending of the limbs and therefore requires less deflection to generate a given output force. This allows shorter limbs to be used.
Another benefit is a reduction in force with increasing flexing to contribute to the variable leverage effect of the rotary elements.
There is a significant negative side effect in routing load cables so as to produce longitudinal flexing of the limbs, i.e., a lack of equality or symmetry of the bending of the limbs. This may occur if the arrow is nocked offcenter on the bowstring. This uneven bending may adversely affect the accuracy of the shot.
This effect is not present to any significant degree in the construction of the Allen patent since the transverse deflection of the bow member limbs is cross leveraged by the load cables and the bowstring. That is, the reaction force tending to require to deflect transversely each limb tip is absorbed by the opposite limb tip, inherently equalizing the bending of tips with each other.
When the load cables are aligned with and anchored to the bow member limbs, no such cross leverage effect is present. Thus, a simple rerouting of the load cables as shown in the Barna patent would inherently produce this undesirable side effect.
In recognition of this, most constructions proposed have included a synchronizing mechanism as in the Hoffmeister patent, U.S. Pat. No. 3,854,467. The synchronizing device disclosed therein comprises an additional synchronizing cable arranged in a "figure eight" around and connected to reels to constrain rotation of the eccentric elements to occur in synchronism with each other. By synchronizing the rotation of the eccentric rotary elements, the bending of the limbs is also equalized.
The addition of this synchronizing mechanism however increases the complexity and weight of the archery bow. The synchronizing mechanism requires a significant space in the handle section of the archery bow and adds weight. It is highly desirable that the complexity and weight of the bow be minimized for sporting use.
Accordingly, it is an object of the present invention to provide a compound archery bow of the type including tip mounted rotary elements to enable a reduced draw force at maximum draw, but which does not require the presence of load cables adjacent to the bowstring in the nocking region.
It is another object of the present invention to provide an archery bow in which there is provided a substantial longitudinal loading of the bow member limbs which does not result in unequal bending of the limbs and does not necessitate the use of a separate synchronizing mechanism.
It is yet another object of the present invention to provide a variable draw force archery bow having a variable draw force rotary elements of minimum weight and complexity.